For a great number of years, golf balls were molded using wound cores, which comprised a soft rubber center surrounded by a layer of thread rubber windings. In the late 1960s to early 1970s, balls with ionomer covers (produced by E.I. du Pont de Nemours and Company, 1007 Market ST Wilmington, Del. 19898 (“DuPont”) under the trade name Surlyn®) were introduced. Balls molded with Surlyn® covers were produced with both thread-wound cores and solid rubber cores. The balls molded using initial grades of Surlyn® and solid cores (hereafter referred to as “two-piece balls”) were significantly less expensive to produce; however, the initial two-piece golf balls were hard, having an unpleasant feel to the golfer.
In the late 1980s, DuPont came out with softer Surlyn® terpolymer grades, known as Very Low Modulus Ionomers (V.L.M.I.). These materials allowed for development of two-piece golf balls with softer covers; however, use of high levels of V.L.M.I. results in a significant detrimental effect on the golf ball resilience. The limitation on balls made with V.L.M.I. materials was (is) that use of high levels of V.L.M.I. materials has a significant detrimental effect on golf ball resilience properties. Therefore, golf balls with soft covers could be made, but had relatively high compression; thus exhibiting high spin rates and low velocity.
In the mid- to late-1990s, softer, i.e. lower compression, distance type golf balls were developed. These golf balls included the addition of an intermediate cover layer. The additional layer allowed for greater control of the performance properties of the golf ball. In the late 1990's, multi-layer golf balls utilizing polyurethane outer covers were introduced. These balls were rapidly adopted by professional golfers due to their premium qualities. However, these balls required a hard feel to achieve the desired distance and spin properties.
Through a softer core, a golf ball molded with a stiff ionomer had a reasonable feel based upon a relatively low compression; however, the core compression can only be reduced to a certain level (a Professional Golfers Association (PGA) while retaining acceptable ball durability. If a core compression of below about 35 was used, impact durability of the golf ball was poor. A favorable byproduct of the use of a soft compression core in a golf ball was a lower spin rate, which allowed for better accuracy of the golf ball.
In 1998, Wilson Sporting Goods Co. (“Wilson”), 8700 West Bryn Mawr Avenue, Chicago, Ill. 60631, introduced a golf ball made using a core with about a 35 compression (sold under the trademark Staff® Titanium Straight Distance). In order to keep the velocity and performance properties of a premium distance golf ball, Wilson used a stiff ionomer cover layer on this ball. The ball compression of this golf ball was approximately 85, which was low for the time when it was introduced.
Existing golf balls, however, have some drawbacks. Prior art golf balls are generally manufactured with a core made primarily from polybutadiene rubber, which is covered with a fairly hard, thin, ionomer inner cover layer, which is subsequently covered by the polyurethane or balata/polybutadiene outer cover layer. While providing adequate playing characteristics at a less expensive production cost, these solid balls exhibit lower velocities at driver impact than wound balls using like cover materials. Prior art golf balls utilized either thermoplastic or thermoset material for the covers. The prior art thermoplastic material allows for greater ease in manufacturing, but reduces resilience. Conversely, thermoset material is difficult with which to work, but provides needed resilience.
In addition, all of the various materials used in the construction of golf balls, from wound core constructions through to multi-layer solid core constructions, have varying densities. Accordingly, the mass per unit volume of these materials varies. For example, typically, the materials used to produce the cover layer possess a lower mass per unit volume than the materials used to produce the core. Additionally, the material composition of most intermediate layers has a density or a weight per unit volume that is different than the density or weight per unit volume of the core and/or the cover layer. If a golf ball is manufactured perfectly, that is if the core or center of a ball is perfectly spherical, and if the cover layer thickness and intermediate layer thickness (if applicable) are constant throughout the entire ball, the ball will be “balanced”, and should fly true when struck with a golf club, or should roll true when putted.
More recently, golf balls have been developed with significantly lower ball compression than was previously considered possible for a premium two-piece golf ball. The Wilson Staff Duo, Callaway Supersoft, and Bridgestone Extra Soft (produced by Bridgestone Sports Co., LTD., Omori Bellport E Bldg. 6-22-7, Minami-oi Shinagawa-ku, Tokyo 140-0013 Japan) have all been introduced in recent years, having compression ranging from about 40 to about 65. These balls are designed to produce low ball compression through the use of softer and larger cores, and softer cover materials (ionomer blends comprising varying levels of V.L.M.I. materials). These golf balls produce soft feel and reasonable distance performance, but are generally low spin and do not produce great control around the green.
Even more recently, low compression balls comprising three-layer construction have been developed. The Wilson Staff Duo Spin and Bridgestone e6 produced by Bridgestone Sports Co., LTD., Omori Bellport E Bldg. 6-22-7, Minami-oi Shinagawa-ku, Tokyo 140-0013 Japan) were developed having compression in the range of about 40 to about 60. The 3-piece balls provide distance performance with the added benefit of improved performance around the green. However, construction has been limited such that compression of less than about 40 has been difficult to achieve with acceptable performance and impact durability.